Long-range laser radar (ladar) systems for ranging, imaging, tracking, and targeting applications need highly spatially and spectrally coherent continuous wave (CW) output or short pulses of high-power, near diffraction limited, beams with high spatial coherency and near transform limited spectral content for accuracy and long range capabilities. Typical platforms for such ladar systems include high-performance aircraft, spacecraft, weapons systems, or portable equipment in which space is limited, but power output, beam quality, and beam control requirements are paramount. There are also many other applications for optical amplifiers, lasers, beam transport systems, and beam input/output and switching controls that are efficient, low mass, and compact in size, yet can be scaled to high average as well as peak powers while producing a high quality, near diffraction limited, beam. For example, laser beams are used extensively in industry for materials processing, cutting, and drilling applications and in medical surgical procedures in which very narrowly focused, high intensity beams produce sharper, cleaner cuts.
A common TEM00 beam is one type of beam in which the light energy is spatially coherent (same phase across the thickness or cross-section of the beam) and is the lowest spatial mode of a laser. (Spatial mode in context of spatial coherency refers to the degree to which the laser is spatially coherent and should not be confused with modes of light transmission or propagation in a waveguide, which are also discussed herein.) A TEM00 beam has a Gaussian amplitude distribution and can be focused down to the smallest size—much more so than higher modes, thus concentrating the light energy in the beam to a high intensity. A TEM00 beam can also be propagated for long distances with minimal spreading or expansion of beam size. For many applications, therefore, it is desirable to pack as much energy as possible into TEM00 beams. For example, for cutting materials, packing more energy into a TEM00 beam means more power that can be focused to a very small spot to cut better, sharper, and cleaner than, a higher mode, e.g., TEM01, or TEM10, that has less spatial coherency of the light energy in the beam.
For laser radar (ladar) detection of range (distance away), velocity (speed and direction of travel), and even shapes or images of objects, such as targets, from long distances away, high average power CW lasers or pulses of TEM00 beams are preferred for minimizing power loss of beams propagated over such long distances due to beam spreading, scattering, and attenuation in the atmosphere. Further, to maximize the likelihood that light reflected by the target back to the ladar receiver will still be strong enough to be detected in the midst of all other light energy of similar wavelengths in the atmosphere (background noise), which also reaches the range detector, the launched light beam should have a high level of energy. However, if there are several targets or objects close to each other, a long pulse will not allow the range detector to distinguish between light energy reflected from the several targets or objects, respectively, unless high bandwidth amplitude modulated (AM) or frequency modulated (FM) chirp is utilized. Such range discrimination, i.e., the minimum distance separating two reflective surfaces that can be detected separately, is even more critical in laser imaging applications in which the range detector must be able to discriminate between different reflecting surfaces of the same object or target in order to determine its shape. Such imaging along with range detection may be used, for example, to distinguish between an enemy tank and an adjacent house or to determine if an airplane has the shape of a commercial airliner or a military bomber. Therefore, to detect and image targets at the longest range (distance away), beams of short, high-power, pulses with near diffraction limited, spatial coherency and near transform limited spectral content are most effective, although use of high bandwidth AM, FM, or phase modulated (PM) light followed by an optimized matched filter receiver is also very effective in many applications.
Unfortunately, prior to this invention, typical adverse, non-linear and thermal effects, such as thermal self-focusing and self-phase modulation, stress birefringence, stimulated Rayleigh, Raman, and Brillouin scattering, intermodel dispersion, and the like, limit the amount of power that can be produced by current state-of-the-art waveguide resonators and amplifiers, such as those that use crystalline laser materials in the form of bulk rods or slabs pumped with laser diodes and non-crystalline materials such as glassy optical fibers. However, at high powers, it is difficult to achieve both excellent pump/mode matching with high pump absorption and diffraction-limited beam quality. Longitudinal pumping can result in excellent mode matching, but it is limited in power due to the thermal stress fracture limit, i.e., the medium will crack when it gets too hot [S. Tidwell et al., “Scaling CW Diode-End-Pumped Nd:YAG Lasers to high Average Powers,” IEEE J. Quantum Electron, vol. 28, 997 (1992)].
Another common problem in state-of-the-art bulk laser geometries prior to this invention is thermal management—both in the form of heat extraction and dissipation as well as optical distortion due to thermal gradients. The heat build-up results from absorption of high pump energy in a small volume of laser material, and active cooling in the form of bulky heat exchangers or refrigeration systems is usually required to remove the heat. Such active cooling adds severely to the size, weight, and power requirements of the system. Thermal gradients in the laser materials are manifested in the forms of undesirable thermal lensing or self-focusing, due to thermally-induced birefringence, which alters polarization of the light. See, for example, David Brown, “Nonlinear Thermal Distortions in YAG Rod Amplifiers”, IEEE J. Quantum Electron, vol. 34, 2383 (1998). Considerable research has been devoted to compensation schemes for these adverse thermal effects. These problems are significant, because there is typically power dependent birefringence, which alters polarization, and bi-focusing, which degrades spatial and temporal coherence. See, James Sherman, “Thermal compensation of a CW-pumped Nd:YAG laser”, Appl. Opt., vol. 37, 7789 (1998). One technique that has been tried to alleviate this effect is to use extremely thin laser media (“thin disks”) such that thermal gradient is reduced and one-dimensional. See U. Branch et al., “Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm”, Opt. Lett., vol. 20, 713 (1995). However, operation in quasi-three-level laser material (Yb, Er, Tm, Ho) severely exacerbates the thermal problem, since it requires much higher pumping to reach threshold and/or refrigerated cooling to depopulate the thermal laser level. Consequently, there has not been any real solution to the thermal problems when scaling bulk laser materials to high power levels.
Optical fiber lasers and amplifiers overcome some of the thermal problems of bulk laser crystal materials by greatly increasing the length of the gain medium and providing mode confinement, i.e., limiting the size of the fiber core diameter so that it can only propagate the lowest order eigenmode, (so-called “single-mode fibers”). There are several benefits to this approach, including: (i) the long interaction length between the pump light and the laser beam lead to high gain and efficient operation, even in 3-level lasers in which the terminal laser level is thermally populated; (ii) Heat is distributed over a longer length of laser medium with a larger surface area, so the heat can be dissipated with passive conductive cooling to the atmosphere or to a heat sink; (iii) Operation can be restricted to a single transverse mode, which preserves a TEM00 spatial coherence and Gaussian intensity profile for the beam focusability and beam propagation with minimal beam spreading advantages as described above; (iv) The flexible nature of the optical fibers allows compact and novel optical designs; (v) The optical fibers can be directly coupled to other passive or active waveguides for modular functionality; and (vi) Fabrication is suited to large-scale production, which reduces costs. However, power scaling, i.e., scaling up to higher power levels, in such single-mode optical fibers is restricted by inability to make efficient coupling of pump light energy into the optical fiber and by the minute, single-mode core, (10–30 μm diameter) holey or photonic fibers and core cluster fibers, which can only handle so much light energy without overheating, distortion, or suffering catastrophic facet (coupling surface) failure.
This limitation of fiber lasers and amplifiers has been partly overcome by use of a double-clad fiber structure in which the small-diameter, single-mode core is surrounded by an inner cladding region, which, in turn, is surrounded by an outer cladding region. The inner cladding region has a larger numerical aperture than the core, thus can accept more pump light energy in more modes. Therefore, the pump light is optically confined to both the core and inner cladding regions together, while the optical beam is confined to the core alone. However, drawbacks of such double-clad fiber designs for laser resonators and amplifiers include: (i) The pump light energy, while introduced into, and confined by, the core and inner cladding together, is absorbed only in the core region so that the effective absorption coefficient is reduced by approximately the ration of the core area to the inner cladding area; (ii) The inner cladding size is still very small, even though larger than the core, so that coupling of a laser diode array into the inner cladding region is still quite difficult and not very efficient; and (iii) The outer cladding region must be made with a much lower index of refraction than the inner cladding for optical confinement of the pump light to the inner region, and such lower index of refraction materials are often polymers (plastic), which are much more susceptible to damage than glass, especially from heat.
Essentially, the single-mode or large effective area core diameter of optical fibers is so small (10–30 μm, which is equivalent to 7.8×10−7 cm2 in cross-sectional area) that a 10 μJ (micro joule) pulse of light has a fluence (energy per unit area) greater than 13 J/cm2 (joules per square centimeter), which is close to the damage threshold of the fiber. Larger core diameter can handle more energy, of course, so that a 10 μJ pulse of light would not be so close to the damage threshold, but larger core diameters result in undesirable eigenmode mixing and resulting loss of polarization, spatial coherence, and temporal coherence, which are significant beam degradations that reduce usability and effectiveness of the beam and should be avoided. Some complex-design, large-area, multi-mode fibers have been reported with reduced mode-mixing and pulse energies up to 500 μJ with M2<1.2, (M2 is a measure of divergence relative to diffraction limit and M2=1 is diffraction limited) have been reported [see, e.g., H. Offerhaus et al., “High-energy single-transverse mode Q-switched fiber laser based on multimode large-mode-area erbium-doped fiber”, Opt. Lett., vol. 23 (1998)], but no truly single-mode (LP01) fiber design has been able to break the nanosecond-class, short pulse, 1 mJ (1,000 μJ) barrier, while maintaining spectral and spatial coherence with short temporal pulse widths.
In many applications, including those addressed by this invention, production and amplification of high-power, high quality laser and other light beams is only part of the problem. Transporting such high-power, high-quality beams to points of application, such as the industrial cutting and materials processing, medical, laser radar ranging, imaging, and tracking applications mentioned above, can also present heretofore unsolved problems. For example, in the laser radar (ladar) system described in U.S. Pat. No. 5,835,199, which is incorporated herein by reference, a high-power laser beam is produced for launching from airplanes or other platforms for ranging, imaging, and tracking objects or targets as much as twenty miles away or more. In an airplane, the most effective launch point for such high-power beams may be in the nose cone or in other extremities of the airplane where space is tight and where many electronic and other kinds of equipment also have to fit. Consequently, it is often not possible to place high power laser beam production and amplifying equipment at the most effective launch locations in the airplane. Therefore, it would be very beneficial to have some way of transporting high power laser beams from some other location in the airplane to one or more launch points in the nose cone, fuselage floor, wings, tail, or other structures without degrading beam power, quality, polarization, and the like, and to have an effective way of directing or steering such high power beams at such remote launch points for the best overall ranging, targeting, or imaging results.
Similar beam transport capabilities would also be beneficial in industrial, medical, imaging, directed energy, and other applications of high power laser and other light beams, where space is limited or where it would just be more convenient to place a high powered, high quality beam without all the associated beam production and/or amplification equipment.
Yet, transport of high power, high quality laser beams without degradation of beam power, quality, temporal and spatial coherence, polarization, and the like presents serious problems with many of the same kinds of obstacles as described above for the beam production and amplification. For example, single-mode waveguides, such as single-mode optical fibers, can maintain beam quality, but their very small size for single-mode operation severely limits power transport capabilities. Industrial medical, and even imaging applications would benefit from continuous wave (cw) output power of 100 watts or more, while even higher power laser applications, such as Q-switched or pulsed lasers, may have output power in the megawatt range, such as 10 megawatts or greater. Single-mode waveguides, including fibers, are simply unable to handle that kind of optical power or light energy.
Multi-mode fibers and waveguides are larger than single-mode fibers and waveguides, thus can transport more power, but they do not maintain spatial coherence, polarization, and the like, because of multi-mode interference and other reasons mentioned above. Free-space light transport has its own problems, not the least of which is that the light paths have to be unobstructed and alignment and stability problems in non-laboratory environments are extremely difficult to overcome and are often insurmountable.
Techniques have been previously developed to actively compensate for finite length circular fiber spatial mode deficiencies, including SBS phase conjugation, but these techniques are limited in scope to narrow spectral line width lasers to match the SBS gain bandwidth, enough optical power to provide the nonlinear drive field required, and wavefronts that are not fully reconstructed by phase conjugation. Furthermore, and as previously mentioned, it may be desirable in many waveguide applications to maintain polarization. In circular fibers with a uniform index-of-refraction in both the core and cladding, polarization may not be maintained. To preserve polarization, special polarization-maintaining fiber designs may be required which essentially create an asymmetric index difference in orthogonal directions. If this index profile is disturbed, potentially as a result of high power operation, the polarization integrity may drift or be lost.
Beam quality issues may arise, for example, related to mode mixing as previously described, or with regard to “bend, buckle and twist” of the waveguide and potentially resulting modification of at least spatial coherence, wherein, for example, a twist of the waveguide could result in beam formation equivalent to a negative lens, and a bend in the waveguide may result in beam formation potentially equivalent to a positive lens. Such applications of waveguide technology have not been adequately addressed in the past attempts previously described or in other previous beam transport technologies.